Airbags that deploy in the event of a vehicle crash are an important part of the overall safety system of an automobile. Airbags in combination with seatbelts and other safety systems reduce death and injury that can occur during a vehicle crash. However, in certain circumstances an airbag can present a hazard to a vehicle occupant if the vehicle occupant is positioned too close to the airbag as the airbag deploys. In such a circumstance the vehicle occupant is described as “out of position.” If a vehicle occupant is “out of position” with respect to an airbag, it is better if the airbag does not deploy, or if deployment has begun, that deployment be terminated by venting the airbag. One known approach is to monitor the position of the vehicle occupant using sensors located within the passenger compartment and to use safety system logic to not deploy an airbag in those situations where the vehicle occupant is out of position. This approach has several difficulties: there is a time delay between when the decision to deploy is made and when the actual deployment takes place; and during this time delay the vehicle occupant may have moved closer to the airbag, and may be out of position during actual airbag deployment. Sensors located within the passenger compartment must operate in a crash environment where noise, debris, and electromagnetic interference may make reliable detection of a vehicle occupant's position difficult. Sensors located within the passenger compartment are typically required to be on at all times which consumes power.
Another approach to dealing with the “out of position” problem is to mount a sensor on the inside or outside of the airbag. One type of sensor is mounted inside the airbag and uses tapes sewn to the inside of the airbag, which are drawn out of tape cartridges passing by sensors which monitor the rate at which the airbag deploys by monitoring the rate at which tape is withdrawn from the cartridges. Examples of such systems are disclosed in EP 0990 567, and EP 0 812 741 and commonly owned U.S. patent applications Ser. No. 10/359257 published as 2004/0155445 A, Ser. No. 10/369697 published as 2004/0164533 A, and Ser. No. 10/382538 published as 2004/0174156 A which are incorporated herein by reference. If the rate of tape withdrawal slows down before the airbag has fully deployed, that is an indication that the airbag has collided with an object before full deployment, and the airbag can be vented. In order to obtain a better understanding of airbag deployment dynamics, a means for determining when the airbag deployment into the passenger compartment has begun is needed.